The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Liquefaction of large volumes of gas using a refrigerant circuit is energy and capital intensive. Broadly speaking, a plant for liquefying natural gas comprises a main heat exchanger in which a hydrocarbon gas feed stream is liquefied by means of indirect heat exchange with evaporating refrigerant in one or more stages. The plant further comprises a refrigerant circuit in which evaporated refrigerant(s) are compressed, cooled and returned to the main heat exchanger. The refrigerant circuit typically includes a compressor train consisting of at least one compressor body driven by means of a mechanical driver (e.g., a gas turbine, a steam turbine, or an electric motor) that is connected to the shaft of the compressor body via a common shaft or via a gearbox. The configuration of compressors and mechanical drivers in a gas processing plant greatly influences the energy efficiency of the plant.
New gas liquefaction and other gas processing plants are being designed for ever-increasing production rates in order to realize the favourable economic benefits associated with larger plants. These larger plants have larger refrigeration duties with higher refrigerant circulation rates, and therefore larger refrigerant compressors are required. The amount of natural gas which can be cooled per unit of time in the refrigerator is proportional to the volumetric flow rate of the refrigerant through the refrigerator. However, there is a practical upper limit to the volumetric flow rate which can be handled by a single large compressor with the result that the maximum achievable production rate is being limited by the maximum available compressor sizes.
At the present time, the largest available single compressor bodies suitable for mixed refrigerant service are limited to suction volumetric flows of about 250,000 m3/hr. This limit has contributed to LNG train capacities remaining below 5.5 million tonnes per annum. In order to overcome this limitation, several approaches have been employed or proposed. One approach has been to increase the number of separate refrigerant cycles from two to three. Another approach is to duplicate the entire mixed refrigerant compression system using two complete parallel compression strings, with each string being arranged on a separate shaft using its own driver system. Using this approach, the entire mixed refrigerant compression flow is shared between the two parallel compression strings in order to overcome the limitation imposed by the maximum volumetric flow rate which can be handled by a single compressor body. The parallel string concept can be done in two different ways. One way is to duplicate the compressors only, then mix the flow through the other equipment and split the flow before it passes through the compressors. The other way is to have two unconnected (but identical) mixed refrigerant circuits, having two cooling systems and two main exchangers as well as two compressors. The use of parallel strings may be preferred to a system with a third separate refrigerant cycle, since if one of the parallel strings is shut down, the refrigeration cycle can continue to operate at a reduced capacity using the remaining string, thus maximising the availability of the plant in the event of a compressor outage. However, the duplication inherent in the use of parallel strings may increase the capital cost of a gas processing plant.
Other prior art refrigerant compression circuits are described in International patent publication number WO 01/44734; U.S. Pat. No. 3,527,059; U.S. Pat. No. 6,691,531; U.S. Pat. No. 6,637,238; U.S. Pat. No. 6,691,531; and, U.S. Pat. No. 6,962,060.
There exists a continuing need in the gas processing field to provide alternative plants and methods to eliminate the limitations on the size and volumetric flow of single large compressors without the use of parallel strings for all stages of compression. The present invention overcomes or ameliorates at least one of the aforementioned drawbacks of the prior art to provide an alternative method for the design of a refrigerant compression circuit for a hydrocarbon liquefaction plants.